1. Field of the Invention
This invention relates to the field of voltage regulators, and particularly to high efficiency, high speed, low noise regulators suitable for powering diode lasers.
2. Description of the Related Art
One type of voltage regulator is an amplifier which receives an input voltage as a desired value and provides a regulated output voltage which follows the input voltage. A linear mode voltage regulator is a voltage regulator which works in linear mode; i.e., the output stage of the regulator has continuous current flowing between two transistor pairs. Linear mode regulators have the advantages of high precision, low noise, and fast response speed—i.e., a wide bandwidth from input to output.
However, such regulators have a low efficiency when the voltage drop across the regulator is high, such as 0.5V to 5V, which happens often, especially when driving laser diodes. Because of this, a linear mode regulator cannot provide a high average output current or power without generating a considerable amount of heat. Although a rail-to-rail linear regulator can provide high output current without generating much heat in its saturation (or near saturation) region, the output current can no longer be regulated (or will soon be unregulated), and the regulator will stop or will soon stop functioning.
A switch mode regulator is one in which a switching circuit switches the output voltage between negative (often the ground) and positive power supply rails, and an output inductor and capacitor network filter out the switching pulses, resulting in a smoothed and continuous output voltage, which is regulated by a control circuit through changing the duty cycle of the pulses in order to follow an input voltage. Such regulators are typically capable of providing a large continuous output current with high power efficiency; as such, they can provide a high average output power without generating much heat. Switch mode regulators can also be configured to produce an output voltage which is higher than the positive power supply rail (the boost type), or lower than the negative rail (the inverting type). These two functions cannot be realized by linear regulators, since their output stages will be saturated when the outputs approach the power rails.
However, switch mode regulators tend to have a low response speed—i.e., a narrow signal bandwidth from input to output, and large noise. The low response speed is caused by the output filtering circuit, which includes at least one inductor in series with the load, and a capacitor in parallel with the load. The large noise is caused by the pulse-width modulation (PWM) control circuit, which provides a series of pulses with their duty cycles adjusted as needed to operate the switching circuit to regulate the output voltage.